Airfoils constructed with spanwise cavities and passageways for carrying coolant fluid therethrough are well known in the art. Cooling fluid is brought into the cavities; and some of the fluid is ejected via holes in the airfoil walls to film cool the external surface of the airfoil. The trailing edge region of airfoils is generally difficult to cool because the cooling air is hotter when it arrives at the trailing edge since it has been used to cool other portions of the airfoil. The relative thinness of the trailing edge region makes it more susceptible to damage due to overheating and thermal stresses.
In U.S. Pat. No. 4,303,374 the pressure side wall of the airfoil terminates short of the trailing edge formed by the suction side wall (i.e. the pressure side wall is "cut back") thereby exposing the inside surface of the suction side wall in the trailing edge region to the hot gases passing around the airfoil. A spanwise slot in the trailing edge region discharges cooling fluid from a central cavity over the exposed inside surface of the suction side wall. Disposed within the trailing edge slot are a plurality of partitions which are spaced apart in the spanwise direction defining transverse cooling flow channels therebetween within the trailing edge slot. Each partition has an upstream portion with straight, parallel side walls, and a downstream portion which tapers to substantially a point at the outlet of the slot. The transverse channels, therefore, include a straight upstream portion and a diffusing downstream portion. The object is to form a continuous sheet of cooling air which remains attached to the exposed inside surface of the suction side wall downstream of the slot outlet. Other patents showing spanwise trailing edge region slots and cut back pressure side walls are U.S. Pat. Nos. 3,885,609; 3,930,748; and 4,229,140.
It is also known to provide straight (as opposed to tapered) ribs along the exposed inside surface of the suction side wall downstream of the trailing edge slot for carrying cooling fluid from the slot across that exposed portion.
In the art of cooling turbine blades of gas turbine engines, it is important to minimize the amount of coolant flow required to cool the blades, because that cooling air is working fluid which has been bled from the compressor, and its loss from the gas flow path reduces engine efficiency. It is also desirable to cut back the pressure side wall of turbine airfoils to improve airfoil aerodynamics; however, this results in a trailing edge region which is likely to be too thin to accommodate an internal cavity with conventional film cooling holes extending outwardly therefrom. Instead, spanwise trailing edge region slots and cut back pressure side walls have been used in place of conventional film cooling holes, such as shown in hereinbefore discussed U.S. Pat. No. 4,303,374.
In airfoils with thin trailing edge regions, the cut back portion of the trailing edge is film cooled by cooling air exiting from a slot within the trailing edge region. The cooling air exiting the slot forms a film on the exposed internal surface of the suction side wall downstream of the slot. To be effective, decay of the film as it moves further downstream from the slot outlet must be minimized to the extent that the film is still sufficiently effective at the trailing edge. In this specification and appended claims, the distance between the cut back downstream edge of the pressure side wall and the trailing edge of the airfoil as defined by the suction side wall downstream end is the "cut back distance" x. The longer the cut back distance x the more difficult it is to maintain film cooling effectiveness over the full length of the cut back.
Despite the variety of trailing edge region cooling configurations described in the prior art, further improvement is always desireable in order to allow the use of higher operating temperatures, less exotic materials, and reduced cooling air flow rates through the airfoils, as well as to minimize manufacturing costs, such as by being able to cast the entire airfoil, including all cooling air channels. Presently in high temperature blades, the channels within the trailing edge slot are very thin and are machined, such as by electro discharge machining, using thin, rod-like electrodes. Casting requires larger passageways, which can result in the airfoil becoming too thin in the trailing edge. Also, wider channels may flow too much cooling fluid if incorporated into airfoils in a conventional manner.